The amount of CO2 in the lower atmosphere has increased from 280 to 400+ parts per million over the last 40 years.
Via isotopic analysis, scientists know that this increase in CO2 is due to human pollution.
Carbon dioxide is a greenhouse gas that traps heat in the lower atmosphere. As a result of this trapped heat, the planet is warming.
This, of course, is widely understood except by some people with weak scientific backgrounds, or scientists paid for by the oil industry.